Using the South African frog, Xenopus, molecular investigations will be undertaken of the role of particular DNA sequences in two modes of genetic interaction: genetic recombination and transposition. These are ubiquitous natural processes which result in rearrangements of genetic information. It is proposed to investigate their basic mechanisms and to evaluate their handling of specific DNA sequences. It has been demonstrated that bacteriophage Lambda DNAs injected into Xenopus oocytes engage in apparently normal crossing over. Further studies are designed to elucidate the mechanism of recombination by physical characterization of intermediates and products, by establishment of a cell-free recombination system which can be fractionated, and by examining the process at various stages of oogenesis. In addition, specially designed Lambda vectors will be used to study the recombination behavior of specific Xenopus DNA sequences. Of particular interest are satellite 1 DNA, which is suspected to be relatively inactive in recombination, and 5S DNA, which may have a recombination hotspot within its spacer. Two families of Xenopus transposable elements have been identified. These will be characterized further by nucleotide sequence analysis and Southern blot-hybridization. We already know that these elements differ in significant ways from transposable elements in other species, and their unique features will be carefully documented. Special emphasis will be placed on a sub-class of hybrid PTR-Tx2 elements in which the Tx2 member may be getting a free ride. Comparison of elements between Xenopus species will relate to possible functions and/or evolutionary mechanisms. An attempt will be made to demonstrate transposition of elements injected into oocytes or eggs; if successful, this system will be used to investigate the transposition mechanism.